Superconductivity was first observed by the Dutch physicist H. K. Onnes in 1911 during his investigations of the electrical conductivities of metals at very low temperatures. He observed that as purified mercury is cooled, its electrical resistivity vanishes abruptly at a temperature of 4.16.degree. K. Above this temperature, the electrical resistivity is small but finite and measurable; alternatively, when the temperature is reduced below 4.16.degree. K., the electrical resistivity is so small that it is effectively zero. This distinct temperature at which the transition and loss of effective electrical resistivity occurs has been termed the critical temperature or "T.sub.c ". Onnes believed he had discovered a new state of physical matter at temperatures below the critical temperature and coined the term "superconducting state" for the observed phenomenon at temperatures below the critical temperature (T.sub.c) and the term "normal state" for the electrical properties observed at temperatures above the critical temperature. Onnes also found that the superconducting transition is reversible and that the superconducting material recovered its normal, electrical resistivity at the critical temperature.
The modern theory of superconductivity is the result of the research investigations by Bardeen, Cooper, and Schrieffer [Phys. Rev. 106:162 (1957)]. Their proposal, conventionally known as the "BCS theory", has now gained universal acceptance because it has proved capable of explaining most of the observed phenomena relating to superconductivity. Their principles employ a quantum mechanical treatment of the superconductive phenomenon; and their theory has been employed to explain the various observable effects such as zero electrical resistance, the Meissner effect, and the like. Since the BCS theory is so steeped in quantum mechanics, the reader is directed to published texts in the scientific literature for a complete description and explanation. These include: M. A. Omar, Elementary Solid State Physics: Principles And Applications, Addison-Wesley Publishing Company, 1975, pages 496-527; M. Tinkham, Introduction To Superconductivity, McGraw-Hill Co., 1975.
Superconductivity has been found not to be a rare phenomenon It is exhibited by a substantial number of atomic elements, metallic alloys, and most recently, refractory oxide ceramics. For many years, the highest known critical temperature was only 23.degree. K. There has, accordingly, been intense interest and research investigations into finding superconductive materials with much higher critical temperatures, most desirably those which hopefully would approach room temperature (20.degree. C.) Until very recently, efforts to achieve this goal have met with complete failure. Beginning about 1986, however, polycrystalline scintered ceramic pellets of yttrium-barium-copper oxide and mixtures of potassium, barium, bismuth, and oxygen without copper have been found to demonstrate relatively high critical temperatures (T.sub.c) and superconductivity at temperatures up to 120.degree. K. [Bednorz, J. G. and K. A. Muller, Z. Phys. B64:189 (1986); Wu et al., Phys. Rev. Lett. 58:905 (1987); and Chu et al., Phys. Rev. Lett. 60:941 (1988)]. These investigations and the development of everhigher T.sub.c superconductive materials continues to be an area of intense exploration.
A concomitant and continuing problem has also arisen regarding the electrical joining of superconductive materials, particularly high T.sub.c superconductors to themselves and to other electrically conductive materials in the normal state. By definition, electrically conductive materials in the normal state include both the normal conductors such as gold, silver, copper, and iron; and the semi-conductors such as carbon, silicon, gray tin, and germanium as well as their mixtures with indium, gallium, antimony, and arsenic.
Traditionally, solders--a general term for alloys useful for joining metals together by the process of soldering--have been used to electrically join conductors to themselves and to semi-conductors. The principal types of solder conventionally known are: soft solders such as lead-tin alloys; and brazing solders such as alloys of copper and zinc and sometimes silver. Representative of conventionally known solders and soldering techniques are U.S. Pat. No. 3,600,144 describing a low melting point brazing alloy; U.S. Pat. No. 4,050,956 describing a method of chemically bonding metals to refractory oxide ceramics; U.S. Pat. No. 4,580,714 disclosing a hard solder alloy comprising copper, titanium, aluminum, and vanadium; U.S. Pat. No. 4,582,240 revealing a method for intermetallic diffusion bonding of piezo-electric components; U.S. Pat. No. 4,621,761 identifying a brazing process for forming strong joints between metals and ceramics while limiting the brazing temperature to not more than 750.degree. C.; and U.S. Pat. No 4,631,099 describing a method for adhesion of oxide type ceramics and copper or a copper alloy. More recent attempts to refine techniques for lowering the resistance of electrical contacts between superconductive materials include annealing bulk sintered samples of yttrium-barium-copper oxide at temperatures up to 500.degree. C. for an hour [Superconductor News, May--June, 1988, page 5]; and the use of laser energy to deposit a thin film of superconductive yttrium-barium-copper oxide directly onto a silicon substrate [Superconductor News, May--June, 1988, page 1]. All of these methods require either extreme temperatures or sophisticated equipment. It is, thus, unequivocally clear, therefore, that there remains a long recognized and continuing need for new compositions able to serve as solders and which demonstrate little or no electrical resistance at superconductive temperatures and yet are able to maintain strength and adhesion between superconductors or superconductive materials with conductors and semi-conductors.